1. Field of the Invention
The present invention relates to a high frequency module incorporating a layered substrate including a plurality of dielectric layers and a plurality of conductor layers alternately stacked.
2. Description of the Related Art
Recently, cellular phones operable in a plurality of frequency bands (multibands) have been put to practical use. Front-end modules in which switching between transmission signals and reception signals is performed by a switch circuit are known as front-end modules incorporated in cellular phones that conform to the time division multiple access system and that are operable in a plurality of frequency bands. Such front-end modules are called antenna switch modules or high frequency switch modules, for example. In the present patent application, a combination of circuits performing processing of high frequency signals and a substrate for integrating these circuits, including such front-end modules, is called a high frequency module. In a high frequency module, for example, a layered substrate including a plurality of dielectric layers and a plurality of conductor layers alternately stacked is used as the substrate.
In a high frequency module having such a layered substrate, there are cases in which some of circuit components are formed using some of the conductor layers located inside the layered substrate, and at least one element as another circuit component is mounted on the top surface of the layered substrate. Furthermore, in such a high frequency module, there are cases in which a plurality of terminals for connection to external circuits are disposed on the bottom surface of the layered substrate. In such a high frequency module, when it is necessary to connect two of the circuit components to each other, they are connected to each other using through holes provided in the layered substrate or in the conductor layers inside the layered substrate, for example. When it is necessary to connect the at least one element mounted on the top surface of the layered substrate to the terminals provided on the bottom surface of the layered substrate, they are connected to each other using through holes provided in the layered substrate, for example.
JP 2002-217656A discloses a multilayer substrate including a power supply line for supplying a direct-current power to transistors. In this publication, the power supply line incorporates an inductor. This inductor is formed using a plurality of strip conductor traces inside the multilayer substrate. The front end and the terminal end of each of the strip conductor traces are respectively short-circuited and connected to separate terminals. In addition, each of the strip conductor traces is short-circuited by a via (through hole) at an arbitrary point between the front end and the terminal end.
JP 2000-059113A discloses a transmission line having such a configuration that a plurality of strip-shaped line electrodes are stacked with dielectric layers respectively disposed between adjacent ones of the line electrodes, and the line electrodes are connected through via holes (through holes).
In a high frequency module for processing high frequency signals, through holes on a signal transmission line each have an inductance. As a result, such a high frequency module incorporating a signal transmission line including through holes has a problem that the characteristics of the high frequency module vary in response to the magnitude of an inductance produced because of the through holes.
In a case in which an element mounted on the top surface of the layered substrate is connected to a terminal disposed on the bottom surface of the layered substrate by a signal transmission line including through holes, in particular, the inductance of the signal transmission line increases since the transmission line is long. Here, in many cases the element mounted on the top surface of the layered substrate is designed so that impedance matching is obtained between the element and an external circuit connected thereto via the terminal. In such cases, if the element mounted on the top surface of the layered substrate is connected to the terminal disposed on the bottom surface of the layered substrate by the signal transmission line including the through holes, there may arise a problem that it is impossible to achieve the impedance matching between the element and the external circuit because of the inductance resulting from the through holes.
Furthermore, in a case in which a plurality of circuit components are connected to terminals via a signal transmission line including through holes, in particular, the inductance of the signal transmission line is likely to greatly change depending on the configuration of the signal transmission line, and the characteristics of the high frequency module are thereby likely to vary.